3com 5500-ei pwr Installation Instruction

 

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Figure 3-1 BPDU Tunnel network hierarchy 

 

 

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When a BPDU packet coming from a customer network reaches an edge device in the service 

provider network, the edge device changes the destination MAC address carried in the packet from 

a protocol-specific MAC address to a private multicast MAC address, which can be defined using a 

command. A packet with this multicast address as its destination address is called a tunnel packet. 

In the service provider network, the tunnel packet can be forwarded as a normal data packet.  

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Before the device in the service provider network forwards the packet to the destination customer 

network, the edge device will identify the tunnel packet, determine the packet type based on the 

type field in the packet, restore its destination MAC address to the original protocol-specific MAC 

address and then forward the packet to the access device on the user side. This ensures the 

packet to be forwarded is consistent with the packet before entering the tunnel. So, a tunnel here 

acts as a local link for user devices. It enables Layer 2 protocols to run on a virtual local network.  

 and 

 show the structure of a BPDU packet before and after it enter a BPDU tunnel. 

Figure 3-2 The structure of a BPDU packet before it enters a BPDU tunnel 

 

 

Figure 3-3 The structure of a BPDU packet after it enters a BPDU tunnel